The present invention relates to a radiation-curable resin composition, and especially to a radiation-curable resin composition that is suitable as a coating material for substrates, in particular optical fibers.
Optical fibers, especially, optical glass fibers, are fragile and easily damaged and hence these are coated with a coating material. The properties, specifically, the modulus of elasticity and the like of a coating film made of, for example, a UV-ray curable resin have a great influence on the transmission performances of optical fibers. When optical fibers are coated with a primary coating material having a high modulus of elasticity, the transmission loss of the optical fibers increases because of high lateral pressure. A material having a low modulus of elasticity is, therefore, desirable as the primary coating material. However, conventional coating materials having a low modulus of elasticity have the drawbacks that these materials have generally low breaking strength and are hence fragile. Specifically, it is desired that the modulus of elasticity of the primary coating material for optical fibers be 0.15 kg/mm2 or less in terms of secant modulus, as determined according to JIS K7127, to absorb high lateral pressure. However, the breaking strength of cured products having such a low modulus of elasticity is as low as 0.15 kg/mm2 or less in terms of the tensile strength at break, as determined according to JIS K7127. The products thus exhibit only poor practicability.
Furthermore, in spite of excellent transmission characteristics due to the low modulus of elasticity, such conventional radiation curable resin compositions have a drawback when used as the primary coating material for optical fibers. That is, when the primary coating material is removed to connect an optical fiber with another optical fiber, a portion of resinous material is left on the optical fibers. The complete removal of such resinous material remaining requires time-consuming procedures, impairing the processability of the material.
An object of the present invention is to provide a radiation-curable resin composition which is suitable for use as a substrate coating material, in particular, an optical fiber coating material, and, more particularly, as a primary coating material for optical fibers and which, after cure, has a high breaking strength while exhibiting a low modulus of elasticity.
Another object of the present invention is to provide a radiation curable resin composition suitable for use as a primary coating material for optical fibers, which can form a cured coating with a low modulus of elasticity exhibiting a high breaking strength and small light transmission loss, and yet be easily removed from optical fibers.
Accordingly, the present invention provides a coated optical fiber comprising: a coating having, a tensile strength at break of more than 0.15 kg/mm2 and a secant modulus of 0.15 kg/mm2 or less.
Furthermore the present invention provides a radiation curable resin composition comprising:
(A) a polymer containing polymerizable unsaturated groups in an average amount of 1.2 per molecule, a urethane bond in a molecular chain, and having a number average molecular weight from 3,000 to 30,000,
(B) a poly-functional monomer having two or more polymerizable unsaturated groups,
(C) a monomer having one polymerizable unsaturated group, and
(D) a radiation active initiator.
The composition can produce a cured product having at a thickness of 200 xcexcm a secant modulus of 0.15 kg/mm2 or less and a tensile strength of 0.15 kg/mm2 or more when cured by ultraviolet radiation at a dose of 1 J/cm2 in air.
The present invention further provides a radiation curable resin composition, which can produce a cured product having a total of at least two peaks or shoulders in a temperature range from xe2x88x9250xc2x0 C. to +35xc2x0 C., preferably at least one peak or shoulder in a temperature range from xe2x88x9250xc2x0 C. to xe2x88x9220xc2x0 C. and at least one peak or shoulder in a temperature range from xe2x88x9220xc2x0 C. to +35xc2x0 C., in a temperature dependency curve of loss tangent obtained from a temperature dependency measurement of a dynamic viscoelasticity ratio when cured by ultraviolet radiation at a dose of 1 J/cm2 in air. Preferably this composition also comprises the above-mentioned components (A) through (D).